I’m relatively new to WPF, so could anyone give me some guidelines on how to build this UI (it’s main panel of Paint on Windows)

I prefer code style over XAML (just for the learning purpose), so a general idea on what controls should be used would be more than enough, I can handle the detail of each element.
There are online templates. I think that you are searching for Ribbon Control Template.
Then, Codeproject has a great article about this.
You can check this msdn article, too.
In these links you have a lot of examples.
To use it you will create a new project with an online template, like in the image bellow (you see: it’s selected WPF Ribbon Application:
